The wavelength division multiplexing (WDM) system has become popular in the optical communication system. One prior patent application has disclosed an optical transceiver that is implemented with four transmitter optical sub-assemblies (TOSA) each operable in 10 Gbps, four receiver optical sub-assemblies (ROSA) each operable also in 10 Gbps, an optical multiplexer, and an optical de-multiplexer. Such an optical transceiver has realized the transmission speed of 40 Gbps.
FIG. 8A is a plan view of an optical arrangement of components which realizes the optical de-multiplexing function, while, FIG. 8B is a side view thereof. The multiplexed optical signal provided from the external optical fiber 1 passes an optical medium 5 after it is reflected by a mirror 4 and fully reflected by the optical reflector 5a. The optical filter 5b of a type of multi-layered filter transmits only one of optical signals with a specific wavelength λ1 but reflects rest of optical signals at a first portion thereof. Only one of rest of optical signals with another specific wavelength λ2 transmits the optical filter in the second portion thereof after they passes the optical medium 5 and reflected by the reflector 5a. Further, rest of optical signals is reflected by the optical filter 5b in the second portion thereof. The third portion of the optical filter 5b can transmits only one of optical signals with the third specific wavelength λ3, and the fourth portion of the optical filter 5b transmits the rest optical signal with the fourth specific wavelength λ4. Thus, the optical signals with a plural wavelengths, λ1 to λ4, can be de-multiplexed and received by respective photodiodes (hereafter denoted by “PD”) 7a. 
A conventional optical module for the multiplexed optical signals, the optical components such as the optical de-multiplexer, the mirror block, the lenses, the PDs, and so on, are two dimensionally arranged in the base. This arrangement expands the footprint of the components. Recent optical transceivers are forced to make further compact in dimensions thereof. For such requests, the two-dimensional arrangement of the optical components becomes fatal factor to obstruct the installation of the optical module within a compact optical transceiver.